A staring IR-FPA array is an array of radiation detectors that views a scene of interest and that detects thermal (IR) radiation arriving from the scene. This is in contrast to a scanned array wherein incident radiation is scanned over the array through the use of, by example, a rotating mirror. Modern second-generation staring IR-FPA's are capable of providing extremely high dynamic range levels. However, systems that incorporate such sensors can experience a difficulty in realizing the full extent of the IR-FPA's performance. Two main factors that have limited the full realization of the IR-FPA performance are: (a) the system Analog-to-Digital Converter (ADC); and (b) the spatial fixed pattern noise.
The system ADC is used to convert the analog output of each detector unit cell or pixel of the IR-FPA to a digital signal. However, the system ADC is generally limited in resolution (number of output bits) and in conversion speed by power and space constraints. Furthermore, non-uniformities in the IR-FPA output signal limit the ability of a low resolution ADC to capture the IR-FPA's instantaneous dynamic range. In addition, system drift and other effects can introduce spatial non-uniformities in the IR-FPA output. The spatial non-uniformities degrade the image quality, and can dominate the sensitivity of the IR-FPA system.
By example, some currently implemented imaging systems can have as much as 50% of the available instantaneous output range occupied with spatial non-uniformities in the output signal. The desired signal containing usable information sits atop a signal pedestal, and is typically less than 1/100 the size of the pedestal. As a result, such systems require an extremely high resolution (e.g., 14 bits or more) ADC to resolve the usable signal information. As is well known, high resolution ADCs are expensive and consume a significant amount of system power. The high power consumption can make the placement of the ADC at or near a cold focal plane impractical.
FIG. 1A illustrates the full range of charge capability at several stages (unit cell, column, ADC) of the signal path for a conventional IR-FPA with a direct-injection (DI) unit cell and a column-based capacitive transimpedance amplifier (CTIA). As is illustrated, such arrays provide noise levels as low as 700 e- rms at the output and full signal level to 5.times.10.sup.7 carriers (input referred). This places a resolution requirement on the system's ADC that is in excess of 16-bits in order to capture the full dynamic range of the IR-FPA. The high data rates required by these arrays furthermore makes it impractical to implement 16-bit ADCs in most systems; thus 12-bit to 14-bit ADCs are most commonly used.
The spatial non-uniformity in the IR-FPA output signal is typically a result of physical effects in the detector itself and/or in associated readout array, such as R.sub.o A non-uniformities and input offset voltage variations. The non-uniformity can also be a result of system effects, such as undesirable thermal radiation received from the system environment, and scene effects, such as a contrast between the horizon and the terrain. As such, it is often difficult or impossible to remove a significant portion of the spatial non-uniformity by only detector and/or system engineering.